Question

Am trying to write a piece of python code that calculate and print 1000 prime number from 2
however i only got 1999 as the last element in my result. I know there a lot of questions like this asked before but I wanna know why is my code is not working.
btw: how do i declare a boolean value in python? cant find a clue even googled···sad

mylist=[2]
num=1
count=0
while count<1000:
    if num>1:
        add=1
        for i in mylist:
            if num%i==0:
                add=0
                break
        if add==1:
            mylist=mylist+[num]
    num=num+2
    count=count+1
print mylist

Answer 1

Your loop should read while len(mylist) < 1000:, and remove all references to count. Either that, or you should only increment count every time you add a prime to the list.

Also, you don’t ‘declare’ values in Python. You simply assign a value of the desired type to a variable, and poof, you have a variable of that type. In this case, True and False are boolean values and you can assign them to variables.

Here is my version in idiomatic (i.e. written the way an experienced Python programmer might write it) Python:

primes = [2]
candidate = 3
while len(primes) < 1000:
    isprime = True
    for testprime in primes:
        if candidate % testprime == 0:
            isprime = False
            break
    if isprime:
        primes.append(candidate)
    candidate = candidate + 2
print primes

If you want to write it in even terser and faster Python, do this:

import itertools
primes = [2]
candidate_iter = itertools.count(3, 2)
while len(primes) < 1000:
    candidate = candidate_iter.next()
    if all(candidate % testprime != 0 for testprime in primes):
        primes.append(candidate)
print primes